There's been a long standing bug (reported in #1924 and #1933) where custom filenames which contained periods (.) in the template or any token's replacement value would fail to include the default file extension. Although this has already been addressed somewhat by 319eee3, I feel adding the ability to configure whether the extension is included would:

1. Make the behavior clearer to both new and existing users.
2. Allow filename template to include both a custom and the default extension.

I used the export-refactoring branch as a base because it includes modification to files also changed in the PR. If you decide to scrap those changes, let me know, and I'll make a new PR using the master branch as a base.

I currently have Xcode Version 7.2 (7C68) and MacOS X 10.11.2 installed therefore any changes made to ExportDialog.xib required an update to file schema. To help make the merge process easier for you, I've divided this PR into 3 commits. I first updated the schema without making any changes then separately committed the interface and code changes. This should allow you to either:

1. Merge all 3 commits if you don't need to maintain the legacy file schema.
2. Only merge the third commit (code changes) and use the second commit (interface changes) as a guide to make the same changes using an older version of Xcode/OS X.